The author demonstrates the development and treatment of 6 cases of necrosis of the slipped upper femoral epiphysis. In cases of gradual dislocation, the blood supply of the epiphysis remains normal at all stages. Necrosis does not occur spontaneously. In cases of sudden or so-called "traumatic" separation, the supply of blood to the epiphysis is often disturbed, and necrosis may occur spontaneously. Slight disturbance of the blood supply to the epiphysis may be only temporary and complete recovery may take place. Severe damage, however, leads to necrosis. Operative measures to treat the necrosis may lead to an improvement, but complete restoration of necrosis is not possible--as the examples in the article show. Osteoarthritis will follow at an early age in these cases. Thus, to avoid necrosis in cases of sudden dislocation of the epiphysis therapy must be carried out very gently. Closed or operative reduction should not be performed immediately after the slippage. The deformity should be corrected at a later stage.